1. Technical Field of the Invention
The present invention relates to a liquid ejection head such as an ink jet type recording head, and more particularly to a liquid ejection head in which the evaporation of liquid in a liquid passage is suppressed and the viscosity of the liquid is prevented from rising.
2. Description of the Related Art
As a liquid ejection head for generating the variation of pressure in liquid in a pressure chamber to eject the liquid from nozzle openings as droplets, for instance, exemplified are an ink jet type recording head (simply refer it to as a recording head, hereinafter) used for an image recording device such as an ink jet type printer, a color material ejection head used for producing a color filter of a liquid crystal display or the like, an electrode material ejection head used for forming an electrode of an organic EL (Electro Luminescence), an FED (face light emitting display) or the like, a biological organic material ejection head used for producing a bio-chip (a biochemical element), etc.
As one example of the recording head, there is a recording head including a nozzle forming base plate on which rows of nozzles having a plurality of nozzle openings formed are arranged, a passage forming base plate for forming a consecutive passage from a common liquid chamber (reservoir) to the nozzle openings via a pressure chamber, a cavity unit having a seal plate for sealing the openings of the pressure chamber and the common liquid chamber, a piezoelectric element as a pressure generating source capable of generating the variation of pressure in ink in the pressure chamber and a case member to which the piezoelectric element and the cavity unit are attached.
The sealing plate is composed of a composite plate material formed by laminating a PPS (polyphenylene sulfide) resin film as an elastic thin film part on the surface of a metallic support plate such as stainless steel. In the sealing plate, a compliance part for sealing a part of the reservoir is provided. The compliance part is composed only of the elastic thin film part by removing the support plate of an area opposed to the opening surface of the reservoir by, for instance, an etching process. Then, the compliance part functions as a damper for absorbing the variation of pressure in the reservoir when the piezoelectric element is driven.
In the case member, in a part corresponding to the reservoir (the compliance part) on a connecting surface to the cavity unit, a relief recessed part is formed for allowing the deformation of the elastic thin film part due to the variation of pressure in the reservoir. Further, in the case member, atmosphere opening passages are provided through which the relief recessed part communicates with an external part of the head. The atmosphere opening passages are passages for air that allow air in the relief recessed part to be discharged outside or outside air to be introduced to the relief recessed part in accordance with the operation (the deformation of the elastic thin film part) of the compliance part.
When the recording head having the above-described structure is not continuously used for a long period of time, a solvent of ink in the reservoir gradually becomes steam and the steam passes through the elastic thin film and is discharged to an external part through the atmosphere opening passages. Thus, the viscosity of the ink may be possibly increased. The increased viscosity of the ink may possibly cause an inconvenience such as an insufficient discharge.
Thus, to prevent such an inconvenience, a recording head is proposed in which extremely narrow and small control passages that are set to such a passage resistance as to suppress the diffusion of steam are provided between a relief recessed part (a recessed part for a damper) and atmosphere opening passages to allow them to mutually communicate (for instance, see JP-A-2004-148509 (FIG. 4)). The control passage is set to have the transmittance of steam lower than the transmittance of steam of an elastic thin film part by increasing the passage resistance and the passage resistance of the control passage allows the diffusion of the steam of a solvent of ink in a reservoir to be suppressed, so that the viscosity of the ink is prevented from increasing.
In JP-A-2004-148509, the control passages are provided as grooves extending in the planar direction of the sealing plate in an area located outside the ink passages of the pressure chamber or the reservoir (the compliance part) on a connecting surface of the sealing plate to the case member. However, to ensure the length of the control passages necessary for completely exhibiting the transmitting characteristics of the steam, a surplus area for that purpose needs to be provided in the sealing plate. Therefore, the provision of the control passages in the sealing plate restrains the recording head from being miniaturized. Further, when the control passages are provided in a previously limited area, the necessary length of the control passages is hardly ensured.
Further, in the recording head having the above-described structure, a synthetic resin is ordinarily preferably used as a material of the case member in view of easy moldability. On the other hand, the sealing plate of the cavity unit connected to the case member is formed with metal such as stainless steel. Accordingly, a differential thermal expansion exists between these members and a stress is generated in the connecting part owing to the change of temperature or humidity, so that there is a fear that a distortion or separation may possibly arise between the members. Thus, as an adhesive agent for connecting the case member to the cavity unit (sealing plate), a silicon type adhesive agent is preferably employed. Since the silicon type adhesive agent exhibits a flexibility under a connected state, a stress generated in the connecting part can be absorbed and mitigated by the flexibility of the silicon type adhesive agent. Thus, the distortion or the separation between the members can be prevented.
However, in the case of the structure in which the control passages are formed on the connecting surface of the sealing plate to the case member as in JP-A-2004-148509, when the silicon type adhesive agent is used to connect the members respectively, since the adhesive agent has a relatively high fluidity, the adhesive agent may possibly enter the control passages to prevent the functions of the control passages. Therefore, in the invention disclosed in the Patent Document 1, a selectable connecting unit is limited to a connection by transferring, for instance, an epoxy based sheet type adhesive agent. However, the sheet type adhesive agent is not suitable for absorbing and mitigating the stress generated in the connecting part between the members in view of thickness or flexibility.